The past is not concrete. Somebody is monkeying with it, and they just won't leave it alone. I haven't got actual proof of this, but I know one thing: Jellyfish's 1993 album, Spilt Milk, never used to sound the way it sounds now. I mean, sure, it was always a beautifully crafted kaleidoscope blast of Badfinger, Queen and Surf's Up-era Beach Boys. Exactly when, though, did it suddenly become the best album of the Nineties? I kid you not. This is concept LP heaven, the work of a band that could puke out the best Mika song and flush it away without a moment's regret. It hasn't been remastered, so I can only assume that a gang of people - people who'd spent a bit more on their keyboards and listened keenly to the medley at the end of Abbey Road - snuck back and re-recorded my 12-year-old vinyl copy when I wasn't looking.

Spilt Milk is an example of what is becoming an increasingly rare phenomenon: it's a bona fide Overlooked Classic, a Great Lost Album. If we are going to get pedantic, it lacks some of the allegedly quintessential qualities of other Lost Albums. Nobody involved in its creation has - up to the time of writing, anyway - committed suicide. Its master tapes were not destroyed in a studio fire or left in some parents' basement. In fact, Jellyfish even achieved moderate success during their post-Spilt Milk disintegration (success no doubt stifled by dressing like Elton John's more flamboyant cousins at the height of the grunge era). Still, it has that simultaneous whiff, so beloved of record hunters, of under-appreciation and curious longevity, of Not Being Made For Its Times. Even if it has been released on CD. And you can find it for £2.49 used and new. In about three seconds flat. Without having to leave the house.

The Lost Album lover takes what he can get, and doesn't complain. Before civilisation, so the legend goes, man hunted antelope, wildebeest and woolly mammoth. Subsequently, Island Records began to release gatefold vinyl with pink inserts, and he began to hunt that instead. Unlike meat, rare records did not help feed his family or improve his standing with womenfolk (Nick Drake's originals excepted), but who was complaining? As those who blew £15 quid on Bobby Darin's 'folk albums' from the Seventies might tell you, much of the joy was more in the long hours of searching than the often shorter hours of listening.

Whatever you think about the musical character of the Nineties, there has never been - nor will be - another golden age like it for the reissued or reappraised record. If an album had been released in the Sixties or Seventies and not been given due credit at the time, the chances were, between 1992 and 2000, it was going to be rewarded for its patience. This was the era when the world learned to appreciate Nick Drake, Big Star, early Seventies Beach Boys, Laura Nyro, Judee Sill, Tim Buckley, Gene Clark's No Other; when people shook off the punk-led idea that an association with flared trousers was something to be ashamed of. It was a time when a young, possibly hungover person could still walk into a record shop and find an utterly unknown album by some Californians with beards, buy it purely on the strength of those beards and the fact that its sleeve had a picture of a cheeseburger wearing headphones on it, and not know whether, when he got home, he'd be spending the evening with the Crosby, Stills and Nash that could have been, or three future financial analysts on an ill-advised career detour.

We all have different definitions of the Great Lost Album. Mine is probably not as demanding as most. I usually like them to be strangely commercial-sounding and to carry with them a sense that, in a kinder universe, they could have been multi million-sellers. Beards are a plus, as is swirly prog artwork, heavy card and heavy vinyl. Despite these relatively lenient criteria, it is now six years since I last discovered a genuinely great lost album from my own designated golden era (ie 1966-'76), and I've pretty much decided that's my lot.

Oh, I know a few must be still out there. But what do I have to do to get them? All my favourite record shops have closed down, I've never got hooked on online vinyl buying, and I'm pretty sure the hen's tooth original pressing of Stone Angel's debut album isn't lurking behind the Tijuana Brass in my local branch of Save the Children.

'Digging for records was a lot more exciting before the internet,' says Andy Votel, founder of the Finders Keepers record label. He's full of stories of long evenings locked in dealers' flats and money changing hands in car parks. In an era of increasingly slim pickings, he's done surprisingly well, breathing new life into truly lost songs via such compilations as Folk is Not a Four Letter Word and One Nation Under a Grave. But he sees the modern era as a grim one for the lost album hunter. 'When you take the gamble out of record collecting it ceases to be fun,' he says. 'Nowadays anyone with money can be a rare record expert, and that was far from the case 10 years ago.'

The important thing about buying a lost album is the journey, not the destination, but what's even more important is maintaining the illusion that it's very much about the destination. You must tell yourself that you alone, with your instincts, can find rock's Holy Grail, hopefully for £2.79. But who needs instincts when there are Amazon recommended lists and Last FM? Finding a rare album used to feel like a key to a secret knowledge. Now it feels like the key to some slightly tedious, widely known facts on allmusic.com.

Perhaps it's wrong to blame the internet. After all, the lost albums of 1998 should be the found albums of 2008, shouldn't they? Just under a decade ago, the Guardian commissioned me to poll music biz insiders and writers and come up with an alternative all-time top 100 albums chart. The idea was that I 'banned' the likes of Pet Sounds and shed new light on overlooked classics. These days, it reads less like a list of obscure rock critic's favourites and more like the iPod playlist of a self-consciously eclectic estate agent. Words like 'cult' and 'overlooked' have been applied so frequently to the likes of Big Star and Laura Nyro as to have shed all significance. As for Nick Drake, whose Bryter Later and Five Leaves Left came respectively first and fifth in the overlooked chart, he now looms over English folk culture only marginally less omnipresently than The Green Man.

It's not that there isn't a new generation of lost albums; it's more that they don't quite seem so lost, just slightly forlorn. Some of the neo-psychedelic bands from the Elephant 6 label made very pleasant, under-exposed songs in the mid-Nineties. If someone collated the best of them today, it might work as a modern version of the seminal 1970s Nuggets album. But could they ever be proper, corporeal nuggets, when most are already available to download, at the click of a mouse? Imperial Drag's self-titled 1996 debut CD - an album so overlooked that even one of its publicists doesn't remember it - is a spitting, brilliant blast of future glam, but could I ever quite love it like the vinyl copy of Badfinger's Ass that I bought for 49p from Arcade Records in Nottingham in 1995?

Loving a Lost Album is based on a conceit that makes a person believe that, because they have purchased a piece of under-appreciated wax for a bargain price, they have, in small some way, transcended their surroundings. It's the conceit that makes people go crazy for the excellent first album by American Flyer because it's obscure and features a former Velvet Underground member, but get sneery about the second (also excellent, and very similar-sounding) album by Bread because someone's mum once liked it, and it's in Oxfam for £1. If that conceit became a casualty of an age when all music is instantly available for little or no money, then the death of the Lost Album would be acceptable. But the reality is that some Lost Albums fall through the net into a new kind of limbo. Votel bemoans the fact that the internet has not only made digging for records 'more competitive and more expensive than ever' but that it's made major labels even less likely to stump up the money to re-release the overlooked gems in their vaults.

Does Tim Buckley's Blue Afternoon still seem like a lost classic, just because the cheapest you can buy the CD of it online is £70? Not really. It just seems like a perversely overpriced CD. Similarly, as hard as it is to believe that Rare Earth's psych-prog-funk masterpiece One World has yet to be digitally packaged, its aura of lostness is hampered by the fact that a) you'll never find it in a record shop again for under a tenner, and b) its best song has been used on an advert.

Few records fulfil the 'great lost album' criteria on as many levels as Yours Forever More, the debut by Simon Napier-Bell prodigies Forever More. From its slightly accidental existence (Napier-Bell was merely fulfilling a contract), to the two very odd Ringo-ish country filler tracks (real lost albums need eccentric filler!) to Permissive, the grimy forgotten B-movie in which its gnome-like creators starred, to the album's childishly hand-drawn gatefold sleeve, it begs to be preserved exclusively on 12-inch cardboard, preferably with Sellotape in one corner and a former owner's ancient address on the inner side.

Or at least that's what I used to think, back in the days when I would waste afternoons scouring Greater London's record shops for extra copies. These days, its lack of CD reissue just seems slightly cruel and arbitrary. Sucked of their romance and promise, Lost Albums seem lost in a different, sadder way than before. That's perhaps nothing to get too cut-up about. After all, they're only objects. None the less, some music will always improve with age and demand reinvestigation, and it's hard to suppress a shiver when considering the possible future equivalent of today's Lost Albums: unmanned ghost MySpace pages, replete with ancient tour dates and outdated comments from long-withered porn stars inadvertently having their images used to sell ringtones. Who, you find yourself wondering, will take them home, wipe some 20-year-old lint off their grooves, and love them?